One of the charging methods for the portable electronic devices, for example the smart phones and the tablet personal computers, is to utilize a power converter to convert the power into the output voltage. The output voltage is provided to the electronic devices for charging through the transmission line connected to the power converter. FIG. 1 shows a conventional power converter 2, which includes a transformer TX generated an output voltage VOUT at a secondary side of the transformer TX. Resistors R1 and R2 form a voltage divider which is coupled to an output terminal of the power converter 2 to detect the output voltage VOUT and to divide the output voltage VOUT to generate a voltage signal VDIV. A shunt regulator 4 is coupled to the resistors R1 and R2 as well as an optical coupler 6. The shunt regulator 4 will compare the voltage signal VDIV with a reference voltage to determine the current Is flowing through the optical coupler 6. Accordingly, the optical coupler 6 generates a feedback signal Ifb related to the current Is for a control integrated circuit (not shown in FIG. 1) to regulate the output voltage VOUT. There are more and more systems that need to change the output voltage VOUT according to different needs, for example the rapid charging mode or the sleep mode, in order to improve their performance. There are two existing methods for changing the feedback signal Ifb in order to change the output voltage VOUT: changing a divider ratio of the voltage divider to adjust the voltage signal VDIV, or choosing a different reference voltage.
As disclosed by U.S. Pat. No. 5,773,963, one of the methods for changing a divider ratio to adjust the output voltage of a power converter is illustrated in FIG. 2, in which a power circuit 8 converts a power input VAC into an output voltage VOUT for charging a battery 10. The output voltage VOUT is divided by a divider ratio to generate a voltage signal VDIV that is compared with a reference voltage Vref to generate a feedback signal SFB for the power circuit 8 to adjust the output voltage VOUT. A microcontroller 12 controls the switching of a transistor Q1 so as to determine whether a resistor R3 and the resistor R2 should be in a parallel connection. Accordingly, the divider ratio will be controlled and thereby adjust the output voltage VOUT.
On the other hand, U.S. Pat. No. 7,242,339 discloses a method for choosing a reference voltage to adjust the output voltage of a power converter, as shown in FIG. 3, in which a resistor switch circuit 14 is utilized to choose a needed reference Vref in a way that switches Q2 to Q7 in the resistor switch circuit 14 are selectively turned on to set either one of different voltages as the voltage Vp to be divided by several serially-connected resistors, and channel gates Gate 1 to Gate 5 are controlled to choose one of the divided voltage as the reference voltage Vref.
The existing methods for changing the output voltage mainly utilize the switch and the resistor to change the output voltage VOUT. If the adjustable steps of the output voltage VOUT are to be raised, the numbers of the resistor and the switch have to be increased. Adversely, related costs are also increased. Moreover, these switches and resistors can be merely installed out of the integrated circuit (IC), which results in a complicate circuitry.